Virtually all on-highway and off-highway work machines have automatic or manual transmissions with multiple gears. In general terms, gear shifting takes place through disengagement of one clutch, an “offgoing” clutch, coupled with a first gear and close in time engagement of a second clutch, or “oncoming” clutch coupled with a second, higher or lower gear. During shifting, engagement and locking or “ring-in” of the oncoming clutch will often cause a temporary disruption in the output torque of the work machine transmission, in turn changing the rim pull of the work machine wheels and briefly accelerating or decelerating the work machine. Where the oncoming clutch pressure is increased too quickly, for example, a series of surges in output torque as the oncoming clutch alternately locks, then slips, as it settles toward ring-in can make operation uncomfortable for the operator. This phenomenon can also risk jarring or spilling loads carried by the work machine, or unduly stressing various work machine components. Moreover, it is well known that clutches and other transmission components may wear out more quickly than intended where the transmission is subjected to rough shifting. The particularly rough shifting experienced by novice operators is a familiar result of overly rapid clutch pressure increases. Where clutch pressure increases too slowly, excessive clutch slip can occur, resulting in its own set of familiar problems. Many transmissions and shifting methodologies are directed toward providing an optimal profile for clutch pressure modulation.
Shifting “smoothness” can very generally be divided into two categories, acceleration and jerk. Excessive, uncomfortable shifting acceleration can result where the work machine increases or decreases in speed more rapidly than desired. Jerk represents a change in acceleration itself. Over many years of transmission design, engineers have developed numerous designs and shifting strategies to keep acceleration and jerk within tolerable levels. In some instances, however, improvements in shifting smoothness may increase the duration of the shift, reducing operating efficiency. Additionally, where shifting smoothness is achieved by prolonging the duration of the shift, the transmission clutches by definition may slip for longer periods of time, presenting the additional drawback of premature component wear and failure. One common approach is to ramp up oncoming clutch pressure during a shift toward a target pressure calculated to be sufficient to accommodate the torque through the transmission without slipping.
One known shifting method directed to improved shifting quality is discussed in U.S. Pat. No. 5,282,401 to Hebbale et al. Hebbale et al. describe an adaptive control method wherein deviations from certain shifting parameters are stored in a look-up table and subsequently used to determine adaptive corrective values in subsequent shifts, in particular deviations in torque phase attributable to variability in oncoming clutch fill time and/or return spring pressure. In Hebbale et al., oncoming clutch pressure is ramped up to a target pressure, the ramp period being selected as a function of the desired shift feel, the shift type and the clutch involved. The actual ramp rate is selected such that the target pressure will be reached at the end of the ramp period. Hebbale et al. is a relatively complex system, and suffers from certain of the problems associated with other known designs.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.